In general, a construction machine such as an excavator or a wheel loader includes a lower traveling structure and an upper swing structure swingably mounted on the lower traveling structure so that the construction machine can be useful for performing the construction work.
FIG. 1 is a diagrammatic view schematically showing an excavator which is a construction machine that can perform the construction work while an upper swing structure is swiveled with respect to a lower traveling structure.
In a conventional construction machine 1, work apparatuses (or attachments) 4 such as a boom, an arm, and a bucket are provided on an upper swing structure 2 so as to be positioned in proximity to an operator's cab. The work apparatuses 4 and the upper swing structure 2 are driven by hydraulic fluid discharged from a hydraulic pump. For example, the work apparatuses 4 are driven by each actuator including a hydraulic cylinder, and the upper swing structure 2 can be swingably rotated with respect to the lower traveling structure 3 by the rotation of a swing motor 10 during the traveling operation and the construction work.
The upper swing structure as described above can be stopped in rotation by a swing motor brake device. The swing motor brake device is maintained in a braking state at a normal condition, but when the hydraulic pressure from the pilot pump is supplied thereto, the braking state of the swing motor brake device is released.
However, the hydraulic fluid from the pilot pump is partially stored in an accumulator and the remaining hydraulic fluid is returned to a hydraulic tank via a relief valve in the braking state of the swing motor brake device or during the operation of the brake. This causes a problem in that a load continuously occurs at the relief valve, thus resulting in a decrease in the energy efficiency.
Even in the case where a hydraulic circuit is configured such that a hydraulic pressure is used at a traveling speed switching valve, for example, a first or second traveling speed switching valve or a traveling control valve to change the traveling speed in the braking state of the swing motor brake, the load continuously occurs at the relief valve and thus the energy efficiency decreases as described above.